1. Field of the Invention
The present invention relates to a zoom lens, and more particularly to a compact wide angle zoom lens composed of a reduced number of constituent lenses.
2. Related Background Art
Miniaturization and high performance tendency have recently been accelerated especially in the field of super wide angle lens and wide angle zoom lens among interchangeable lenses for 35 mm still camera. Particularly, there are various proposals made for achieving a compact, inexpensive wide angle zoom lens, among which the most suitable type of lens is a zoom lens composed of two lens groups being a negative lens group and a positive lens group. Further, the recent technology employs an aspherical surface to achieve miniaturization, higher performance, and cost reduction of zoom lens. An example of such a lens is disclosed in Japanese Laid-open Patent Application No. 62-94812. In addition, Japanese Patent Publication No. 61-42246 and U.S. Pat. No. 4,560,253 disclose another example of the zoom lens of two lens group type, which is inexpensive and compact in a simple structure with a negative first lens group being composed of a negative lens and a positive lens.
The zoom lens as disclosed in above Japanese Laid-open Patent Application No. 62-94812 is a wide angle zoom lens composed of a reduced number of constituent lenses, which has a field angle of about 8.degree., an f-number of about 4, and a variable power (magnification change) ratio of about 2. In the zoom lens, however, the first lens group is composed of three lenses, which are negative, negative, and positive, respectively, in refracting power, which is insufficient in respect of miniaturization and cost reduction. There is, therefore, a strong desire to have a high performance wide angle zoom lens which is more compact and inexpensive.
To accomplish the miniaturization, there are conceivably possible methods of increasing the refracting power of each lens group and of decreasing the thickness of each lens group. It seems that a limit is reached at present in enhancing the refracting power of each lens group. The remaining method is one of reducing the thickness of each lens group accordingly. The requirements in miniaturization are that a change in total length is small during magnification change and that a lens located at the closest position to an object at a wide angle end has a small lens diameter so as to make a filter size as small as possible, as well as that the total length is shortened. Therefore, the most effective method for achieving the miniaturization and the cost reduction would be to reduce the number of constituent lenses in the first lens group closest to the object while reducing the thickness of the constituent lenses.
However, the zoom lens with the wide field angle of about 84.degree. as disclosed in above Japanese Laid-open Patent Application No. 62-94812 comprises the first lens group composed of at least three lenses even with use of aspherical surface. There is known no wide angle zoom lens with a first lens group being composed of constituent lenses of less than three and with a field angle being of about 84.degree.. A trial was made to simply remove one of the two negative lenses in the first lens group of the conventional wide angle zoom lens, but a preferable result was not obtained because of increased aberrations of distortion, astigmatism, and coma.
On the other hand, above Japanese Patent Publication No. 61-42246 and U.S. Pat. No. 4,560,253 show an example in which the first lens group is composed only of two lenses one of which is a negative lens and the other of which is a positive lens. The zoom lens as disclosed therein, however, has a relatively small field angle of about 54.degree.-62.degree.. Although the zoom lens as disclosed in U.S. Pat. No. 4,560,253 employs the aspherical surface, the field angle is about 62.degree. and it is difficult to further widen the field angle. Additionally, if the refracting power of the first lens group is increased in the arrangement as disclosed, the lens performance is extremely degraded at the wide angle end, revealing a limit of miniaturization at the wide angle end.